The proposed studies are designed to investigate how variations in the composition of plaque and plaque fluid affect the driving force for enamel demineralization and remineralization, and hence caries activity. The proposed studies will provide direct measurement of the physico-chemical factors, microbiological determinants, and the site-specific nature of the caries process. There are five specific aims. Specific Aim l proposes to determine the total concentration of individual organic acids, inorganic ions, and pH in plaque fluid before, and at selected times after, a rinse with a sucrose solution. The fluid will be obtained from plaque associated with sound, white spot lesions and cavitated enamel surfaces obtained from teeth of caries-free (CF) and caries-positive (CP;DMFS>l0) subjects. Specific Aim 2 proposes to study the effect of variations in plaque permeability to fermentable carbohydrate in relation to plaque properties and caries status. Specific Aim 3 proposes to study the effect of the restriction of dietary sucrose intake by human subjects on (a) the microbial and chemical composition of plaque, (b) metabolic activity in plaque after carbohydrate exposure both in vivo and in vitro, (c) the bacterial cell density of plaque, and (d) plaque buffering capacity. Specific Aim 4 proposes to further explore the role of calcium phosphate in plaque and the release of fluoride into plaque, following acid production, as additional cariostatic factors. Specific Aim 5 proposes to conduct in vitro enamel demineralization experiments using demineralizing solutions having compositions similar to plaque fluids associated with cariogenic and non-cariogenic plaques.